Soldering method for mounting semiconductor device on wiring board to ensure invariable gap therebetween, and soldering apparatus therefor

ABSTRACT

In a soldering method for mounting a semiconductor device on a wiring board, a plurality of solid-phase solders s are provided between the semiconductor device and the wiring board, and are thermally melted to thereby produce a plurality of liquid-phase solders therebetween. A constant force is exerted on the liquid-phase solders by relatively moving the semiconductor device with respect to the wiring board so that lo an invariable gap is determined between the semiconductor device and the wiring board.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a soldering method for mounting asemiconductor device on a wiring board, and relates to a solderingapparatus for carrying out such a soldering method.

2. Description of the Related Art

For example, when a flip-chip type semiconductor chip, featuring aplurality of solder bumps as outer electrode terminals, is mounted on aninterposer or wiring board, an invariable gap must always be establishedbetween the semiconductor chip and the wiring board.

Conventionally, in order to ensure the establishment of the invariablegap between the semiconductor chip and the wiring board, a prior artsoldering method utilizes an arrangement of the solder bumps, asdisclosed in, for example, JP-2003-031993 A. Namely, the semiconductorchip is downwardly moved to the wiring board so that the solder bumpsare abutted against on the wring board. When the solder bumps areabutted on the wiring board, a position of the semiconductor chipconcerned is defined as a reference position. Then, while the solderbumps are soldered to the wiring board, the semiconductor chip ispositionally adjusted with respect to the reference position so that anynegative influences, exerted on the establishment of the invariable gapthe semiconductor chip and the wiring board, can be eliminated, asstated in detail hereinafter.

SUMMARY OF THE INVENTION

It has now been discovered that the above-mentioned prior art solderingmethod has a problem to be solved as mentioned hereinbelow.

The solder bumps may fluctuate in size or diameter. When the fluctuationof the solder bumps is large, the prior art soldering method fails toproperly define the reference position, as discussed in detailhereinafter.

In accordance with a first aspect of the present invention, there isprovided a soldering method for mounting a semiconductor device on awiring board. In the soldering method, a plurality of solid-phasesolders provided between the semiconductor device and the wiring boardare thermally molten to thereby produce a plurality of liquid-phasesolders therebetween, and then a constant force is exerted on theliquid-phase solders by moving the semiconductor device with respect tothe wiring board, so that an invariable gap is determined between thesemiconductor device and the wiring board.

The semiconductor device may be moved toward the wiring board so thatthe liquid-phase solders are pressed therebetween. In this case, apressing force exerted on the liquid-phase solders is detected duringthe relative movement of the semiconductor device with respect to thewiring board, and the relative movement of the semiconductor device iscontrolled so that the pressing force is obtained as a constant force,resulting in the determination of the invariable gap between thesemiconductor device and the wiring board. The relative movement of thesemiconductor device toward the wiring board may be carried out during arise in temperature of the solid-phase solders.

On the other hand, the semiconductor device may be moved away from thewiring board so that the liquid-phase solders are stretchedtherebetween. In this case, a pulling force exerted on the liquid-phasesolders is detected during the relative movement of the semiconductordevice with respect to the wring board, and the relative movement of thesemiconductor device is controlled so that the pulling force is obtainedas the constant force, resulting in the determination of the invariablegap between the semiconductor device and the wiring board. The relativemovement of the semiconductor device away from the wiring board may becarried out during a fall in temperature of the solid-phase solders.

Preferably, the semiconductor device is held by a driver unit having aload sensor, and is moved with respect to the wiring board by drivingthe driver unit having the load sensor. In this case, a force exerted asa reaction force on the driver unit by the liquid-phase solders may bedetected by the load sensor so that the force is obtained as theconstant force, resulting in the determination of the invariable gapbetween the semiconductor device and the wiring board. Also, the forcemay be detected as a pressing force exerted on the liquid-phase solders.Optionally, the force may be detected as a pulling force exerted on theliquid-phase solders.

In accordance with a second aspect of the present invention, there is asoldering apparatus for mounting a semiconductor device on a wiringboard. The soldering apparatus includes a stage on which the wiringboard is placed, and a driver unit that holds the semiconductor device.The semiconductor device is moved with respect to the wiring board bythe driver unit so that a constant force is exerted on a plurality ofliquid-phase solders provided between the semiconductor device and thewiring board, whereby an invariable gap is determined between thesemiconductor device and the wiring board.

The soldering apparatus may further comprise a load sensor that detectsa force which is exerted as a reaction force on the semiconductor deviceby the liquid-phase solders, and a control unit that controls the driverunit so that the force is obtained as the constant force. In this case,the force may detected as a pressing force obtained by moving thesemiconductor device toward the wiring board. Optionally, the force isdetected as a pulling force obtained by moving the semiconductor deviceaway from the wiring board.

In the soldering apparatus, the load sensor may be lo contained in thedriver unit so that the force is detected as one exerted on the driverunit by the semiconductor device. The force may be detected as apressing force exerted on the liquid-phase solders. Optionally, theforce may be detected as a pulling force exerted on the liquid-phasesolders. Preferably, the load sensor features a resolution ability of atmost 0.02 N.

In accordance with a third aspect of the present invention, there isprovided a soldering method for mounting a semiconductor device on awiring board. In this third aspect, a semiconductor device having aplurality of external metal terminals is held by a driver unit, and thesemiconductor device is placed on a wiring board by the driver unit sothat the external metal terminals are provided therebetween. Then, theexternal metal terminals are thermally heated to thereby produce meltedmetal terminals, and the semiconductor device is relatively moved withrespect to the wiring board by the driver unit. Then, a force exerted asa reaction force on the driver unit by the melted metal terminalsbetween the semiconductor device and the wiring board is detected duringthe relative movement of the semiconductor device with respect to thewiring board, and the driver unit is controlled so that the force isobtained as a predetermined constant force, resulting in determinationof an invariable gap between the semiconductor device and the wiringboard.

In accordance with a fourth aspect of the present invention, there isprovided a soldering apparatus for mounting a semiconductor device on awiring board. In the fourth aspect, the soldering apparatus includes astage on which the wiring board is placed, a driver unit that holds thesemiconductor device having a plurality of external metal terminals, sothat the semiconductor device is relatively moved with respect to thewiring board, and a heater unit that thermally melts the external metalterminals. Also, the soldering apparatus includes a load sensor thatdetects a force exerted as a reaction force on the semiconductor deviceby the melted external metal terminals between the semiconductor deviceand the wiring board during the relative movement of the semiconductordevice with respect to the wiring board, and a control unit thatcontrols the driver unit so that the force is obtained as apredetermined constant force, resulting in determination of aninvariable gap between the semiconductor device and the wiring board.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more clearly understood from thedescription set forth below, as compared with the prior art method, withreference to the accompanying drawings, wherein:

FIGS. 1A through 1F are partial cross-sectional views for explaining aprior art soldering method for mounting a semiconductor chip on a wiringboard;

FIGS. 2A and 2B are partial cross-sectional views, corresponding toFIGS. 1A and 1B, for explaining how to define a reference position of asemiconductor chip with respect to a wiring board;

FIGS. 3A and 3B are partial cross-sectional views, corresponding toFIGS. 2A and 2B, for explaining why a predetermined gap between thesemiconductor chip and the wiring board cannot be obtained;

FIG. 4 is a schematic view of an embodiment of a soldering apparatusaccording to the present invention;

FIGS. 5A to 5C are explanatory views for explaining an operationalprinciple of the soldering apparatus of FIG. 4;

FIGS. 6A and 6B are other explanatory views for explaining theoperational principle of the soldering apparatus of FIG. 4;

FIG. 7 is another explanatory view for explaining the operationalprinciple of the soldering apparatus of FIG. 4;

FIG. 8 is a flowchart of a soldering routine executed in the controlunit of FIG. 4;

FIG. 9 is a flowchart of a first example of a gap determination routineexecuted in step 804 of FIG. 8;

FIGS. 10A to 10D are explanatory views for explaining the first exampleof the gap determination routine of FIG. 9;

FIG. 11 is a flowchart of a second example of the gap determinationroutine executed in step 804 of FIG. 8; and

FIG. 12 is a flowchart of a third example of the gap determinationroutine executed in step 804 of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before a description of an embodiment of the present invention, forbetter understanding of the present invention, a prior art solderingmethod for mounting a semiconductor chip on a wiring board will beexplained with reference to FIGS. 1A through IF. Note, the prior artsoldering method is disclosed in, for example, JP-2003-031993 A, and iscarried out by using a soldering apparatus.

First, referring to FIG. 1A which is a schematic partial cross-sectionalview, a soldering apparatus, which is partially illustrated, isindicated by reference numeral 1. The soldering apparatus 1 includes anX-Y stage 11, and a tool head or chip-holder head 12 provided above theX-Y stage 11. Also, the soldering apparatus 1 includes a control unit(not shown) containing a microcomputer to control operations of the X-Ystage 11, and the chip-holder head 12 and so on.

A wiring board, generally indicated by reference numeral 2, is prepared,and is set in place on the X-Y stage 11. The X-Y stage 11 is operatedunder control of the control unit (not shown) so that the wiring board 2is moved in an X direction and a Y direction perpendicular to eachother.

A wiring board 2 includes an insulating substrate 21 composed of aplurality of insulating layers in each of which an interconnectionpattern structure (not shown) is formed, a plurality of electrode pads22 formed in the uppermost insulating layer of the insulating substrate21, and a plurality of provisional solders 23 with which the respectiveelectrode pads 22 are coated.

For example, for the solder, it is possible to use a suitable alloycomposed of tin (Sn), silver (Ag) and copper (Cu), and each of theelectrode pads 22 is composed of copper (Cu), gold (Au) or the like,exhibiting a wettability by a thermally melted solder. Also, theuppermost insulating layer of the insulating substrate 21 is defined asa solder resist layer, exhibiting a non-wettability by a thermallymelted solder, which may be composed of polyimide resin, epoxy resin orthe like.

Also, as shown in FIG. 1A, a semiconductor chip, generally indicated byreference numeral 3, is prepared, and is held by the chip-holder head12, which may be constructed as a vacuum sucker to suck and hold thesemiconductor chip 3. The chip-holder head 12 is operated under controlof the control unit so as to be vertically moved with respect to the X-Ystage 11.

The semiconductor chip 3 is formed as a flip-chip type semiconductorchip, and includes a semiconductor substrate 31, a plurality ofelectrode pads 32 formed on a top surface of the semiconductor substrate31, and a plurality of solder bumps 33 adhered to the respectiveelectrode pads 32.

Note, there is a mirror image relationship between an arrangement of thesolder bumps 33 and an arrangement of the provisional solders 23.

Each of the solder bumps 33 may be composed of the same alloy as theprovisional solders 34 of the wiring board 2, and each of the electrodepads 32 may be composed of copper (Cu), gold (Au) or the like,exhibiting a wettability by a thermally melted solder. Also, althoughnot illustrated, an uppermost insulating layer of the semiconductor chip3, in which the electrode pads 33 are formed, is defined as a solderresist layer, exhibiting a non-wettability by a thermally melted solder,which may be composed of polyimide resin, epoxy resin or the like.

As shown in FIG. 1A, the wiring board 2 is positioned with respect tothe semiconductor chip 3 by driving the X-Y stage 11 so that therespective solder bumps 32 are vertically aligned with the provisionalsolders 23.

Next, referring to FIG. 1B which is a schematic partial cross-sectionalview, the chip-holder head 12 is downwardly moved until the respectivesolder bumps 33 are abutted against the provisional solders 23.

Next, referring to FIG. 1C which is a schematic partial cross-sectionalview, the provisional solders 23 and the solder bumps 33 are thermallymelted and fused with each other so that a plurality of fused solders FSare respectively produced between the electrode pads 22 and theelectrode pads 32. Then, the fused solders FS are cooled so as to beset, thereby resulting in completion of the mounting of thesemiconductor chip 3 on the wiring board 2.

Note that the X-Y stage 11 and the chip-holder head 12 may containrespective electric heaters to thermally melt the provisional solders 23and the solder bumps 33 for the production of the fused solders FS.Also, note that the soldering apparatus may be provided with a coolingsystem for blowing cool air over the fused solders FS to thereby setthem.

Next, referring to FIG. 1D which is a schematic partial cross-sectionalview, after the setting of the fused solders FS, the semiconductor chip3 is unloaded from the chip-holder head 12, and the semiconductor chip 3with the wiring board 2 is unloaded from the X-Y stage 11.

In the above-mentioned soldering method, although a gap between thewiring board 2 and the semiconductor chip 3 should be always invariable,the gap may fluctuate due to dimensional variations of the X-Y stage 11,the chip-holder head 12 and so on, which are caused by thermal expansionand thermal shrinkage thereof.

JP-2003-031993 A discloses a technique for eliminating the fluctuationof the gap between the wiring board 2 and the semiconductor chip 3. Inparticular, the dimensional variation of the chip-holder head 12 ispreviously measured with respect to a change of a temperature of thechip-holder head 12. During the soldering of the semiconductor chip 3 tothe wiring board 3, the temperature of the chip-holder head 12 isdetected, and a movement of the chip-holder head 12 is controlled by thecontrol unit to thereby compensate for the dimensional variation of thechip-holder head 12, so that the gap between the wiring board 2 and thesemiconductor chip 3 can be maintained constant.

Also, JP-2003-031993 A discloses another technique for ensuring theestablishment of an invariable gap between the wiring board 2 and thesemiconductor chip 3. In particular, when the chip-holder head 12 isdownwardly moved until the solder bumps 33 are abutted against therespective provisional solders 23, and the position of the chip-holderhead 12 is detected and defined as a reference position. Then, duringthe soldering of the semiconductor chip 2 to the wiring board 3, amovement of the chip-holder head 12 is controlled so that thechip-holder head 12 stays at the reference position.

In either event, the controlling of the movement of the chip-holder head12 is based on the reference position, which is renewed whenever asemiconductor chip 3 is mounted on a wiring board 2.

FIGS. 2A and 2B conceptually show how the reference position of thechip-holder head 12 is defined with respect to the wiring board 2.

As shown in FIGS. 2A and 2B which correspond to FIGS. 1A and 1B,respectively, the soldering apparatus 1 further includes a linear scale13 vertically provided along a path for the movement of the chip-holderhead 12, and a scale sensor 14 supported by the chip-holder head 12 todetect and read a division of the linear scale 13. Note, in FIGS. 2A and2B, the scale sensor 14 is conceptually and symbolically represented byan open arrow.

The chip-holder head 12 is downwardly moved from an upper position ofFIG. 2A toward the wiring board 2, and the downward movement of thechip-holder head 12 is continued until the solder bumps 33 of thesemiconductor chip 3 are abutted against the provisional solders 22 ofthe wiring board 2, as shown in FIG. 2B. Namely, when the solder bumps33 are abutted against the provisional solders 22, the downward movementof the chip-holder head 12 is stopped, and a division REF of the linearscale 13 is detected and read by the scale sensor 14. The division REF(see: FIG. 2B) is recognized by the aforesaid control unit as areference position of the chip-holder head 12.

Then, as soon as the provisional solders 23 and the solder bumps 33 arethermally melted, a movement of the chip-holder head 12 is controlled sothat the chip-holder head 12 stays at the reference position until thesoldering of the semiconductor chip 3 to the wiring board 2 iscompleted, whereby an invariable gap can be always obtained between thewiring board 2 and the semiconductor chip 3.

The above-mentioned prior art soldering method is useful provided thatall the solder bumps 33 have no production fluctuation in size, i.e.,that all the solder bumps 33 have the same size as each other. However,for example, when even only one of the solder bumps 33 has a larger sizethan that of the remaining solder bumps 33, the prior art solderingmethod fails in obtaining the invariable gap between the wiring board 2and the semiconductor chip 3.

FIGS. 3A and 3B show why it is impossible to obtain the invariable gapbetween the wiring board 2 and the semiconductor chip 3 when even onlyone of the solder bumps 33 has a larger size than that of the remainingsolder bumps 33.

In FIGS. 3A and 3B which correspond to FIGS. 2A and 2B, respectively,one of the solder bumps 33 shown in FIGS. 2A and 2B is replaced with alarge-sized solder bump indicated by reference 33L. While thechip-holder head 12 is downwardly moved from the upper position of FIG.3A toward the wiring board 2, the large-sized solder bump 33L isprematurely abutted against the corresponding provisional solder 23, asshown in FIG. 3B. At this time, the downward movement of the chip-holderhead 12 is stopped, and a division REF′ of the linear scale 13 isdetected and read by the scale sensor 14.

As shown in FIG. 3B, the division REF′ is shifted from the division REFby a difference between a height of the solder bump 33 and a height ofthe large-size solder bump 33L so that the aforesaid control unitmistakes the division REF for the division REF′. Namely, the divisionREF′ is mistakenly recognized by the aforesaid control unit as thereference position of the chip-holder head 12. Thus, it is impossible toobtain the predetermined gap between the wiring board 2 and thesemiconductor chip 3 when even only one of the solder bumps 33 has alarger size than that of the remaining solder bumps 33.

Incidentally, various methods are well known to produce solder bumps. Ina so-called printing method, it is possible to produce a plurality ofsolder bumps at minimum low cost, but fluctuation in a size of theproduced solder bumps is large. For example, when the solder bumpshaving an aimed size or diameter of 100 μm are produced by the printingmethod, the fluctuation in the aimed size or diameter is ±20 μm. Thus,in the above-mentioned prior art soldering method, it is impossible touse the solder bumps produced by the printing method, for the reasonsalready stated above.

With reference to FIG. 4, an embodiment of a soldering apparatusaccording to the present invention is explained below.

A soldering apparatus, generally indicated by reference 4, includes abase frame 41 fixed on a floor F, and an upright structure 42 securelymounted on the base frame 41. The upright structure 42 includes a columnmember 42A implanted in the base frame 41, an arm member 42Bhorizontally extended from a top end portion of the column member 42A,and a plate-like member 42C extended from a middle portion of the columnmember 42A.

The soldering apparatus 4 also includes an X-Y stage 43 securely mountedon the base frame 41, and the X-Y stage 43 contains an electric heater43A which is symbolically illustrated in FIG. 4. A wiring board 5 is setin place on the X-Y stage 43, and the X-Y stage 43 is operated so thatthe wiring board 5 is moved in an X direction and a Y directionperpendicular to each other.

The soldering apparatus 4 further includes a tool head or chip-holderhead 44 provided above the X-Y stage 43, a load sensor 45 securelymounted on the chip-holder head 44, and a voice coil motor 46 securelysupported by the arm member 42B of the upright structure 42 tovertically suspend both the chip-holder head 44 and the load sensor 45.

The chip-holder head 44 may be constructed as a vacuum sucker to suckand hold a semiconductor chip 6, and contains an electric heater 44Awhich is symbolically illustrated in FIG. 4. Also, the load sensor 45may be formed as a strain gauge featuring a resolution ability of, forexample, at most 0.02 N.

The voice coil motor 46 includes a cylindrical magnet 46A securelyattached to the arm member 46B of the upright structure 42, a solenoid46B movably provided in the cylindrical magnet 46A, and a driven stem46C securely joined to the load sensor 45 so that the verticalsuspension of both the chip-holder head 44 and the load sensor 45 fromthe driven stem 46C is established. Thus, by driving the voice coilmotor 46, both the chip-holder head 44 and the load sensor 45 can bevertically moved with respect to the wiring board 5 mounted on the X-Ystage 43.

In short, the chip-holder head 44, the load sensor 45 and the voice coilmotor 46 form a driver unit for vertically moving the semiconductor chip6 toward and away from the wiring board 5 placed on the X-Y stage 43.

The soldering apparatus 4 further includes a vertical guide rail 47supported by the plate-like member 42C of the upright structure 42, andthe chip-holder head is slidably engaged with the vertical guide rail 47so as to be guided during the vertical movement of the chip-holder head44.

The soldering apparatus 4 also includes a position detector unit 48 fordetecting a vertical position of the chip-holder head 44 during thevertical movement of the chip-holder head 44. In particular, theposition detector unit 48 includes a linear scale 48A verticallyprovided along a path for the movement of the chip-holder head 44, and ascale sensor 48B supported by the chip-holder head 44 to detect and reada division of the linear scale 48A. Note, the linear scale 48A may besupported by a suitable column member (not shown) securely attached tothe base frame 41.

In addition, the soldering apparatus 4 is provided with a control unit49 which contains a microcomputer including a central processing unit(CPU), a read-only memory (ROM) for storing various programs andconstants, a random-access memory (RAM) for storing temporary data, andan input/output (I/O) interface circuit.

The control unit 49 controls an operation of the X-Y stage 44, andelectrically energizes the electric heater 43A contained in the X-Ystage 44. The control unit 49 also controls an operation of thechip-holder head 44, and electrically energizes the electric heater 44Acontained in the chip-holder head 44.

Further, the control unit 49 drives the load sensor 45, and processes aload signal output from the load sensor 45. The control unit 49 alsodrives the solenoid 46B to control an operation of the voice coil motor46. In addition, the control unit 49 drives the position detector unit48, and processes a position signal output from the position detectorunit 48.

With reference to FIGS. 5A, 5B and 5C, an operational principle of thesoldering apparatus 4 according to the present invention is explainedbelow.

First, referring to FIG. 5A which is a partial cross-sectional view, thewiring board 5 is set in place on the X-Y stage 43, and thesemiconductor chip 6 is sucked and held by the chip-holder head 44. Byoperating the X-Y stage 44, the wiring board 5 is positioned so as to bealigned with the semiconductor chip 6.

The wiring board 5 includes an insulating substrate 51 composed of aplurality of insulating layers each having an interconnection patternstructure (not shown), a solder resist layer 52 formed as an uppermostlayer on the insulating substrate 51, a plurality of electrode pads 53formed in the solder resist layer 52, and a plurality of provisionalsolders formed on the electrode pads 53.

For example, for the solder, a suitable alloy, which is composed of tin(Sn), silver (Ag) and copper (Cu), may be used, and each of theelectrode pads 53 is composed of copper (Cu), gold (Au) or the like,exhibiting a wettability by a thermally melted solder. Also, the solderresist layer 52 may be composed of polyimide resin, epoxy resin or thelike, which exhibits a non-wettability by a thermally melted solder.

On the other hand, the semiconductor chip 6 is formed as a flip-chiptype semiconductor chip, and includes a semiconductor substrate 61, asolder resist layer 62 formed as an uppermost layer on the semiconductorsubstrate 61, a plurality of electrode pads 63 formed in the solderresist layer 62, and a plurality of metal bumps or solder bumps 64adhered to the electrode pads 63.

Note, there is a mirror image relationship between an arrangement of thesolder bumps 64 and an arrangement of the provisional solders 54.

Each of the solder bumps 64 may be composed of the same alloy as theprovisional solders 54 of the wiring board 5, and each of the electrodepads 63 may be composed of copper (Cu), gold (Au) or the like,exhibiting a wettability by a thermally melted solder. Also, the solderresist layer may be composed of polyimide resin, epoxy resin or thelike, exhibiting a non-wettability by a thermally melted solder.

In FIG. 5A, the semiconductor chip 6 is downwardly moved toward thewiring board 5 by driving the voice coil motor 46 (see: FIG. 4), and thesolder bumps 64 are abutted against the provisional solders 54. Namely,the solder bumps 64 with the provisional solders 54 are properlyprovided between the wiring board 5 and the semiconductor chip 6.

Then, as shown FIG. 5B, the provisional solders 54 and the solder bumps64 are thermally melted by electrically energizing the electric heaters43A and 44A (see: FIG. 4), so that the provisional solders 54 and thesolder bumps 64 are respectively fused with each other to therebyproduce a plurality of liquid-phase solders LS.

As shown in FIG. 5C, after the production of the liquid-phase soldersLS, the semiconductor chip 6 is further downwardly moved toward thewiring board 5 so that the liquid-phase solders LS are pressed by thesemiconductor chip 6, but the liquid-phase solders LS cannot beimmediately squashed because each of the pressed liquid-phase solders LSproduces a reaction force against a pressing force exerted on theliquid-phase solders LS by the semiconductor chip 6.

Thereafter, the movement of the semiconductor chip 6 is controlled byusing the load sensor 45, so that the pressing force, which is exertedon the liquid-phase solders LS by the semiconductor chip 6, ismaintained at a constant force f₁. When the constant force f₁ is exertedon the liquid-phase solders LS, each of the liquid-phase solders LSproduces a reaction force f₂ against the constant force f₁.

In particular, when the liquid-phase solders LS are pressed by theconstant force f₁, each of the liquid-phase solders LS is deformed so asto be laterally and outwardly swelled as shown in FIG. 5C. At this time,a surface tension acts on each of the pressed liquid-phase solders LS sothat the swelled surface area of each of the liquid-phase solders LS isreturned to the minimum spherical surface area. Namely, the reactionforces f₂ are derived from the respective surface tensions acting on theliquid-phase solders LS.

All the reaction forces f₂ exert on the semiconductor chip 6 as aresultant reaction force f₃ lifting it upwardly. On the assumption thatthe liquid-phase solders LS have the same size, while the movement ofthe semiconductor chip 6 is controlled by using the load sensor 45,i.e., while the resultant force f₃ is balanced with the constant forcef₁, an invariable gap G1 can be always maintained between the wiringboard 5 and the semiconductor chip 6.

In short, if a relationship between the constant force f₁ and theinvariable gap G1 is previously known by either a simulation or a realmeasurement, it is possible to determine the invariable gap G1 bydetecting the constant force f₁ by the load sensor 45 (see: FIG. 4), asstated in detail hereinafter.

Incidentally, for example, when the solder bumps 64 have an aimed sizeor diameter of 100 μm, and when the solder bumps 64 are produced by aprinting method, the fluctuation in the size or diameter is ±20 μm asstated above. Also, it is known that the size data or diameter data ofthe solder bumps 64 represents a Gaussian distribution.

According to the present invention, although the solder bumps 64fluctuate in the size or diameter thereof, it is possible to alwaysdetermine the invariable gap G1 between the wring board 5 and thesemiconductor chip 6.

In particular, referring to FIG. 6A corresponding to FIG. 5A, one of thesolder bumps 64 is replaced with a large-sized solder bump 64L having asize or diameter of, for example, 120 μm, and another one of the solderbumps 64 is replaced with a small-sized solder bump 64S having a size ordiameter of, for example, 80 μm. Note that it is assumed that theremaining solder bumps 64 have the size or diameter of 100 μm.

Next, referring to FIG. 6B corresponding to FIG. 5C, the large-sizedsolder bump 64L and the corresponding provisional solder 54 arethermally fused with each other to thereby produce a large-sizedliquid-phase solder LSL, and the small-sized solder bump 64S and thecorresponding provisional solder 54 are thermally fused with each otherto thereby produce a small-sized liquid-phase solder LSS. Similar to thecase of FIG. 5C, the remaining solder bumps 64 are thermally fused withthe respective provisional solders 54 to thereby produce theliquid-phase solders LS.

Thus, when the liquid-phase solders LS, LSL and LSS are pressed by theconstant force f₁, a reaction force f_(2L) obtained from the large-sizedliquid-phase solder LSL is larger than the reaction force f₂ obtainedfrom each of the liquid-phase solders LS, and a reaction force f_(2S) issmaller than the reaction force f₂ obtained from each of theliquid-phase solders LS. Nevertheless, a resultant reaction force f₃obtained from all the reaction forces f₂, f_(2L) and f_(2S) issubstantially the same one as shown in FIG. 5C, because the differencebetween the reaction forces f₂ and f_(2L) may be compensated with thedifference between the reaction force f₂ and f_(2S). In short, inreality, although the semiconductor chip 6 has the large number ofsolder bumps (64, 64L, 64S) which fluctuate in the size or diameterthereof, the compensation for the reaction forces (f₂, f_(2L), f_(2S))wholly occurs among the liquid-phase solders (LS, LSL, LSS) due to theGaussian distribution of the size or diameter of the solder bumps (64,64L, 64S). Thus, although the solder bumps (64, 64L, 64S) fluctuate inthe size or diameter thereof, the invariable gap G1 can be alwaysdetermined between the wiring board 5 and the semiconductor chip 6.

In the foregoing, although the liquid-phase solders LS are pressed bythe constant force f₁ to thereby determine the invariable gap G1 betweenthe wiring board 5 and the semiconductor chip 6, the liquid-phasesolders LS may be stretched by a constant force f₁′ to thereby determineanother invariable gap between the wiring board 5 and the semiconductorchip 6.

In particular, referring to FIG. 7, after the solder bumps 64 arethermally fused with the respective provisional solders 54 to therebyproduce the liquid-phase solders LS (see: FIG. 5B), the semiconductorchip 6 is upwardly moved away from the wiring board 5 by driving thevoice coil motor 45 (see: FIG. 4), and the movement of the semiconductorchip 6 is feed-back controlled by using the load sensor (see: FIG. 4) sothat a constant force f₁′ is exerted on the liquid-phase solders LS bythe semiconductor chip 6. At this time, the liquid-phase solders LS arestretched by the constant force f₁′ so that a reaction force f₂′ isproduced as a pulling force in each of the stretched liquid-phasesolders LS, with the reaction force f₂′ being also derived from asurface tension acting on the liquid-phase solder LS concerned. All thereaction forces or pulling forces f₃′ exert on the semiconductor chip 6as a resultant reaction force f₃′ pulling it downwardly.

Similar to the above-mentioned case, while the movement of thesemiconductor chip 6 is feed-back controlled by using the load sensor(see: FIG. 4), i.e., while the resultant reaction force f₃′ is balancedwith the constant force f₁′, an invariable gap G2 is maintained betweenthe wiring board 5 and the semiconductor chip 6.

In short, if a relationship between the constant force f₁′ and theinvariable gap G2 is previously known by either a simulation or a realmeasurement, it is possible to determine the invariable gap G2 bydetecting the constant force f₁′ by the load sensor 45 (see: FIG. 4), asstated in detail hereinafter.

Next, referring to FIG. 8 showing a flowchart of a soldering routineexecuted by the control unit 49 of FIG. 4, a soldering method accordingto the present invention is explained below.

At step 801, a chip-loading operation is executed. That is, asemiconductor chip 6 is fed from a chip supply station (not shown) tothe chip-holder head 44 (see: FIG. 4), and is sucked and held by thechip-holder head 44.

At step 802, a wiring-board loading operation is executed. That is, awiring board 5 is fed from a wiring-board supply station (not shown) tothe X-Y stage 43 (see: FIG. 4), and the wiring board 5 is set in placeon the X-Y stage 43.

At step 803, a wiring-board positioning operation is executed so thatthe wiring board 5 is positioned so as to be aligned with thesemiconductor chip 6 by driving the X-Y stage 43, as shown in FIG. 4.

At step 804, a gap determination routine is executed. In the executionof the gap determination routine, the semiconductor chip 6 is solderedto the wiring board 5 so that either of invariable gaps G1 or G2 (see:FIG. 5C or FIG. 7) is determined between the wiring board 5 and thesemiconductor chip 6, resulting in a completion of the mounting of thesemiconductor chip 6 on the wiring board 5.

Note, the gap determination routine is explained as stated in detailhereinafter.

At step 805, a chip-unloading operation is executed so that thesemiconductor chip 6 is unloaded from the chip-holder head 44. Note,after the execution of the chip-unloading operation is completed, thedriving of the voice coil motor 46 is stopped so that the chip-holderhead 44 is returned to the original position (see: FIG. 4).

At step 806, a wiring-board unloading operation is executed so that thewiring board 5 with the semiconductor chip 6 is unloaded from the X-Ystage 43. Thus, the soldering routine ends at step 807.

FIG. 9 shows a flowchart of a first example of the gap determinationroutine executed in the step 804 of FIG. 8.

At step 901, flags F1 and F2 are initialized to be “0”.

Then, at step 902, the chip-holder head 44 is downwardly moved towardthe wiring board 5.

At step 903, it is monitored to determine whether the chip-holder head44 has reached a position at which the respective solder bumps 64 areabutted against the provisional solders 54, as shown in FIG. 10A.Namely, when the solder bumps 64 are abutted against the provisionalsolders 54, the solder bumps 64 with the provisional solders 54 areproperly provided between the wiring board 5 and the semiconductor chip6.

For example, the abutting of the solder bumps 64 against the provisionalsolders 54 may be detected by using the load sensor 45. In particular,after the solder bumps 64 are abutted against the provisional solders54, the load sensor 45 senses a predetermined large load because thedriving of the voice coil motor 46 is continued even though the downwardmovement of the chip-holder head 44 is stopped due to the abutting ofthe solder bumps 64 against the provisional solders 54. Thus, it ispossible to detect the abutting of the solder bumps 64 against theprovisional solders 54 by determining whether the predetermined largeload (e.g., more than 1 kg) is sensed by the load sensor 46.

Optionally, the abutting of the solder bumps 64 against the provisionalsolders 54 may be detected by counting a sufficient time in which thesolder bumps 64 can be abutted against the provisional solders 54 duringthe downward movement of the semiconductor chip 6.

After the abutting of the solder bumps 64 against the provisionalsolders 54 is confirmed, the control proceeds to step 904, in which theelectric heaters 43A and 44A are electrically energized so that theprovisional solders 54 and the solder bumps 64 are thermally melted, sothat the solder bumps 64 are respectively fused with the provisionalsolders to thereby produce the liquid-phase solders LS (see: FIG. 5A andFIG. 10B).

At step 905, a pressing load data L is fetched from the load sensor 45,and the control proceeds to step 906, in which it is determined whetherthe pressing load data L is larger than a predetermined constantpressing force f₁ (see: FIG. 5C). If L≧f₁, the control proceeds to step907, in which the chip-holder head 44 is moved upwardly. On the otherhand, if L<f₁; the control proceeds from step 906 to step 908, in whichthe chip-holder head 44 is moved downwardly.

In either event, the control proceeds to step 909, in which it isdetermined whether the flag F1 is set to be “0” or “1”. At the initialstage, since F1=“0”, the control proceeds to step 910, in which it isdetermined whether a time T₁has elapsed. When the time T₁ has notelapsed, the control returns to step 905.

Namely, the routine comprising steps 906, 907, 908, 909 and 910 isrepeatedly executed until the time T₁ has elapsed at step 910. The timeT₁is previously defined as a sufficient time in which a temperature ofthe liquid-phase solders LS can rise to a soldering temperature (e.g.300° C.) necessary to obtain a sufficient soldering of the semiconductorchip 6 to the wiring board 5, and in which the chip-holder head 44 canstay at a position at which the constant pressing force f₁ is stablyexerted on the liquid-phase solders LS. For example, the time T₁ may bea time falling within a range from 3 to 5 sec.

In short, by repeatedly executing the routine comprising steps 906, 907,908, 909 and 910, the movement of the chip-holder head 44 is feed-backcontrolled so that an invariable gap G1 (see: FIG. 5C) is determinedbetween the wiring board 5 and the semiconductor chip 6.

At step 910, when it is confirmed that the time T₁ has elapsed, thecontrol proceeds to step 911, in which the flag F1 is set to be “1”.

Then, at step 912, it is determined whether the flag F2 is set to be “0”or “1”.

At the initial stage, since F2=“0”, the control proceeds to step 913, inwhich the electric heaters 43A and 44A are deenergized. Then, at step914, a cooling process is started. In the cooling process, cool air isblown over the liquid-phase solders LS, using a well known coolingsystem (not shown) included in the soldering apparatus 4 (see: FIG. 4).

At step 915, the flag F2 is set to be “1”

Then, at step 916, it is determined whether a time T₂ has elapsed. Whenthe time T₂ has not elapsed, the control returns to step 905.

Namely, the routine comprising steps 906, 907, 908, 909, 912 and 916 isrepeatedly executed until the time T₂ has elapsed at step 916. Note, atthis stage, F1=“1” and F2=“1”. The time T₂ is previously defined as asufficient time in which the liquid-phase solders LS can be cooled so asto be set. For example, the time T₂ may be at least 10 sec.

In short, during the cooling process, the movement of the chip-holderhead 44 is feed-back controlled so that the invariable gap G1 (see: FIG.5C) is ensured between the wiring board 5 and the semiconductor chip 6.Accordingly, it is possible to absorb dimensional variations of the X-Ystage 43, the chip-holder head 44 and so on, which are caused by thermalexpansion and thermal shrinkage thereof, and thus the determination ofthe invariable gap G1 can be carried out without being subjected toinfluence from the dimensional variations.

At step 916, when it is confirmed that the time T₂ has elapsed, thecontrol proceeds to step 917, in which the cooling process is stopped.Then, the control returns to step 805 of FIG. 8 by step 918.

FIG. 11 shows a flowchart of a second example of the gap determinationroutine executed in the step 804 of FIG. 8.

At step 1101, flags F1 and F2 are initialized to be “0”. Then, at step1102, the chip-holder head 44 is downwardly moved toward the wiringboard 5.

At step 1103, it is monitored to determine whether the chip-holder head44 has reached a position at which the respective solder bumps 64 areabutted against the provisional solders 54 (see: FIG. 10A). Namely, whenthe solder bumps 64 are abutted against the provisional solders 54, thesolder bumps 64 with the provisional solders 54 are properly providedbetween the wiring board 5 and the semiconductor chip 6.

Note that the abutting of the solder bumps 64 against the provisionalsolders 54 may be detected in substantially the same manner as in thecase of FIG. 9.

After the abutting of the solder bumps 64 against the provisionalsolders 54 is confirmed, the control proceeds to step 1104, in which theelectric heaters 43A and 44A are electrically energized so that theprovisional solders 54 and the solder bumps 64 are thermally melted, sothat the solder bumps 64 are-respectively-fused with the provisionalsolders to thereby produce the liquid-phase solders LS (see: FIG. 5A andFIG. 10B).

At step 1105, the chip-holder head 44 is upwardly moved. Then, at step1106, a pulling load data L′ is fetched 15 from the load sensor 45, andthe control proceeds to step 1107, in which it is determined whether thepulling load data L′ is smaller than a predetermined constant pullingforce f₁′ (see: FIG. 7). If L′≦f₁′, the control proceeds to step 1108,in which the chip-holder head 44 is moved downwardly. On the other hand,if L′>f₁′, the control proceeds from step 1107 to step 1109, in whichthe chip-holder head 44 is moved upwardly.

Note, when the chip-holder head 44 is upwardly moved as soon as theelectric heaters 43A and 44A are electrically energized, the solderbumps 64 of the semiconductor chip 6 may be separated from theprovisional solders 54 of the wiring board 5, but the solder bumps 64can be again abutted against the provisional solders 54 because thepulling load data L′ is detected as zero from the load sensor 45 duringthe separation of the semiconductor chip 6 from the provisional solders54.

In either event, the control proceeds to step 1110, in which it isdetermined whether the flag F1 is set to be “0” or “1”. At the initialstage, since F1=0, the control proceeds to step 1111, in which it isdetermined whether a time T₁ has elapsed. When the time T₁ has notelapsed, the control returns to step 1106.

Namely, the routine comprising steps 1106, 1107, 1108, 1109, 1110 and1111 is repeatedly executed until the time T₁ has elapsed at step 1111.The time T₁ is previously defined as a sufficient time in which atemperature of the liquid-phase solders LS can rise to a solderingtemperature (e.g. 300° C.) necessary to obtain a sufficient soldering ofthe semiconductor chip 6 to the wiring board 5, and in which thechip-holder head 44 can stay at a position at which the constant pullingforce f₁′ is stably exerted on the liquid-phase solders LS. For example,the time T₁ may be a time falling within a range from 3 to 5 sec.

In short, by repeatedly executing the routine comprising steps 1106,1107, 1108, 1109, 1110 and 1111, the movement of the chip-holder head 44is feed-back controlled so that an invariable gap G2 (see: FIG. 7) isdetermined between the wiring board 5 and the semiconductor chip 6.

At step 1111, when it is confirmed that the time T₁ has elapsed, thecontrol proceeds to step 1112, in which the flag F1 is set to be “1”.Then, at step 1113, it is determined whether the flag F2 is set to be“0” or “1”,

At the initial stage, since F2=“0”, the control proceeds to step 1114,in which the electric heaters 43A and 44A are deenergized. Then, at step1115, a cooling process is started. In the cooling process, a cool airis blown over the liquid-phase solders LS, using the well known coolingsystem.

At step 1116, the flag F2 is set to be “1” Then, at step 1117, it isdetermined whether a time T₂ has elapsed. When the time T₂ has notelapsed, the control returns to step 1106.

Namely, the routine comprising steps 1106, 1107, 1108, 1109, 1110, 1113and 1117 is repeatedly executed until the time T₂ has elapsed at step1117. Note, at this stage, F1=“1” and F2=“1”. The time T₂ is previouslydefined as a sufficient time in which the liquid-phase solders LS can becooled so as to be set. For example, the time T₂ may be at least 10 sec.

In short, during the cooling process, the movement of the chip-holderhead 44 is feed-back controlled so that the invariable gap G2 (see: FIG.7) is ensured between the wiring board 5 and the semiconductor chip 6.Accordingly, it is possible to absorb dimensional variations of the X-Ystage 43, the chip-holder head 44 and so on, which are caused by thermalexpansion and thermal shrinkage thereof, and thus the determination ofthe invariable gap G2 can be carried out without being subjected toinfluence from the dimensional variations.

At step 1117, when it is confirmed that the time T₂ has elapsed, thecontrol proceeds to step 1118, in which the cooling process is stopped.Then, the control returns to step 805 of FIG. 8 by step 1119.

FIG. 12 shows a flowchart of a third example of the gap determinationroutine executed in the step 804 of FIG. 8.

At step 1201, the chip-holder head 44 is downwardly moved toward thewiring board 5. Then, at step 1202, it is monitored to determine whetherthe chip-holder head 44 has reached a position at which the respectivesolder bumps 64 are abutted against the provisional solders 54 (see:FIG. 10A). Namely, when the solder bumps 64 are abutted against theprovisional solders 54, the solder bumps 64 with the provisional solders54 are properly provided between the wiring board 5 and thesemiconductor chip 6.

Note that the abutting of the solder bumps 64 against the provisionalsolders 54 may be detected in substantially the same manner as in thecase of FIG. 9.

After the abutting of the solder bumps 64 against the provisionalsolders 54 is confirmed, the control proceeds to step 1203, in which theelectric heaters 43A and 44A are electrically energized so that theprovisional solders 54 and the solder bumps 64 are thermally melted, sothat the solder bumps 64 are respectively fused with the provisionalsolders to thereby produce the liquid-phase solders LS (see: FIG. 5A andFIG. 10B).

At step 1204, division data D on the linear scale 48A is fetched fromthe position detector unit 48. Then, at step 1205, the division data Dis stored as reference position data RE in the RAM of the control unit49.

At step 1206, division data on the linear scale 48A is again fetchedfrom the position detector unit 48. Then, at step 1207, it is determinedwhether the division data D concerned is larger than the referenceposition data RE.

If D≧RE, the control proceeds to step 1208, in which the chip-holderhead 44 is moved downwardly. On the other hand, if D<RE, the controlproceeds from step 1207 to step 1209, in which the chip-holder head 44is moved upwardly.

In either event, the control proceeds to step 1210, it is determinedwhether a time T₁ has elapsed. When the time T₁ has not elapsed, thecontrol returns to step 1206.

Namely, the routine comprising steps 1206, 1207, 1208, 1209 and 1210 isrepeatedly executed until the time T₁ has elapsed at step 1210. The timeT₁ is previously defined as a sufficient time in which a temperature ofthe liquid-phase solders LS can rise to a soldering temperature (e.g.300° C.) necessary to obtain a sufficient soldering of the semiconductorchip 6 to the wiring board 5. In short, the semiconductor head 6 staysat the position, at which the solder bumps 64 are abutted againstprovisional solders 54, until the temperature of the liquid-phasesolders LS can rise to the soldering temperature (e.g. 300° C.).

At step 1210, when it is confirmed that the time T₁ has elapsed, thecontrol proceeds step 1211, in which the chip-holder head 44 is upwardlymoved. Then, at step 1212, the electric heaters 43A and 44A aredeenergized, and, at step 1213, a cooling process is started. In thecooling process, cool air is blown over the liquid-phase solders LS,using the well known cooling system.

At step 1214, a pulling load data L′ is fetched from the load sensor 45,and the control proceeds to step 1215, in which it is determined whetherthe pulling load data L′ is smaller than a predetermined constantpulling force f₁′ (see: FIG. 7). If L′≦f₁′, the control proceeds to step1216, in which the chip-holder head 44 is moved downwardly. On the otherhand, if L′>f₁′, the control proceeds from step 1215 to step 1217, inwhich the chip-holder head 44 is moved upwardly.

In either event, the control proceeds to step 1218, in which it isdetermined whether a time T₂ has elapsed. When the time T₂ has notelapsed, the control returns to step 1214.

Namely, the routine comprising steps 1214, 1215, 1216, 1217 and 1218 isrepeatedly executed until the time T₂ has elapsed at step 1218. The timeT₂ is previously defined as a sufficient time in which the liquid-phasesolders LS can be cooled so as to be set. For example, the time T₂ maybe at least 10 sec. Then, the control returns to step 805 by step 1220.

In short, during the cooling process, the movement of the chip-holderhead 44 is feed-back controlled so that the invariable gap G2 (see: FIG.7) is ensured between the wiring board 5 and the semiconductor chip 6.Accordingly, similar to the case of FIG. 11, it is possible to absorbdimensional variations of the X-Y stage 43, the chip-holder head 44 andso on, which are caused by thermal expansion and thermal shrinkagethereof, and thus the determination of the invariable gap G2 can becarried out without being subjected to influence from the dimensionalvariations.

In the above-mentioned embodiments, although the electrode pads 53 ofthe wiring board 5 are coated with the respective provisional solders54, it is possible to eliminate the provisional solders 54 from theelectrode pads 53, if necessary.

Also, in the above-mentioned embodiments, although the flip-chip typesemiconductor chip 6 featuring the solder bumps 64 is soldered to andmounted on the wiring board or interposer 5, the present invention maybe applied to a case where a semiconductor package, such as a ball gridarray (BGA) package or the like, featuring a plurality of outerelectrode terminals or solder balls, is soldered to and mounted on aprinted wiring board.

Further, although the provisional solders 54 and the solder bumps 64 arethermally melted by the electric heaters 43A and 44A, it is possible tocarry out the thermal melting thereof by using hot air.

Finally, it will be understood by those skilled in the art that theforegoing description is of preferred embodiments of the devices andmethods, and that various changes and modifications may be made to thepresent invention without departing from the spirit and scope thereof.

1. A soldering method for mounting a semiconductor device on a wiringboard, which soldering method comprises: thermally melting a pluralityof solid-phase solders provided between said semiconductor device andsaid wiring board, to thereby produce a plurality of liquid-phasesolders therebetween; and exerting a constant force on said liquid-phasesolders by relatively moving said semiconductor device with respect tosaid wiring board, so that an invariable gap is determined between saidsemiconductor device and said wiring board.
 2. The soldering method asset forth in claim 1, wherein said semiconductor device is relativelymoved toward said wiring board so that said liquid-phase solders arepressed therebetween.
 3. The soldering method as set forth in claim 2,further comprising: detecting a pressing force exerted on saidliquid-phase solders during the relative movement of said semiconductordevice with respect to said wiring board; and controlling the relativemovement of said semiconductor device so that said pressing force isobtained as said constant force, resulting in the determination of theinvariable gap between said semiconductor device and said wiring board.4. The soldering method as set forth in claim 2, wherein the relativemovement of said semiconductor device toward said wiring board iscarried out during a rise in temperature of said solid-phase solders. 5.The soldering method as set forth in claim 1, wherein said semiconductordevice is moved away from said wiring board so that said liquid-phasesolders are stretched therebetween.
 6. The soldering method as set forthin claim 5, further comprising: detecting a pulling force exerted onsaid liquid-phase solders during the relative movement of saidsemiconductor device with respect to said wring board; and controllingthe relative movement of said semiconductor device so that said pullingforce is obtained as said constant force, resulting in the determinationof the invariable gap between said semiconductor device and said wiringboard.
 7. The soldering method as set forth in claim 5, wherein therelative movement of said semiconductor device away from said wiringboard is carried out during a fall in a temperature of said solid-phasesolders.
 8. The soldering method as set forth in claim 1, wherein saidsemiconductor device is held by a driver unit having a load sensor, andis moved with respect to said wiring board by driving said driver unithaving said load sensor, and wherein a force exerted as a reaction forceon said driver unit by said liquid-phase solders is detected by saidload sensor so that said force is obtained as-said constant force,resulting in the determination of the invariable gap between saidsemiconductor device and said wiring board.
 9. The soldering method asset forth in claim 8, wherein said force is detected as a pressing forceexerted on said liquid-phase solders.
 10. The soldering method as setforth in claim 8, wherein said force is detected as a pulling forceexerted on said liquid-phase solders.
 11. A soldering apparatus formounting a semiconductor device on a wiring board, which solderingapparatus comprises: a stage on which said wiring board is placed; and adriver unit that holds said semiconductor device, wherein saidsemiconductor device is relatively moved with respect to said wiringboard by said driver unit so that a constant force is exerted on aplurality of liquid-phase solders provided between said semiconductordevice and said wiring board, whereby an invariable gap is determinedbetween said semiconductor device and said wiring board.
 12. Thesoldering apparatus as set forth in claim 11, further comprising: a loadsensor that detects a force which is exerted as a reaction force on saidsemiconductor device by said liquid-phase solders; a control unit thatcontrols said driver unit so that said force is obtained as saidconstant force.
 13. The soldering apparatus as set forth in claim 12,wherein said force is detected as a pressing force obtained by movingsaid semiconductor device toward said wiring board.
 14. The solderingapparatus as set forth in claim 12, wherein said force is detected as apulling force obtained by moving said semiconductor device away fromsaid wiring board.
 15. The soldering apparatus as set forth in claim 12,wherein said load sensor is contained in said driver unit so that saidforce is detected as one exerted on said driver unit by saidsemiconductor device.
 16. The soldering method as set forth in claim 15,wherein said force is detected as a pressing force exerted on saidliquid-phase solders.
 17. The soldering method as set forth in claim 15,wherein said force is detected as a pulling force exerted on saidliquid-phase solders.
 18. The soldering method as set forth in claim 15,wherein said load sensor features a resolution ability of at most 0.02N.
 19. A soldering method for mounting a semiconductor device on awiring board, which soldering method comprising: holding a semiconductordevice having a plurality of external metal terminals by a driver unit;placing said semiconductor device on a wiring board by said driver unitso that said external metal terminals are provided therebetween;thermally heating said external metal terminals to thereby producemelted metal terminals; moving said semiconductor device with respect tosaid wiring board by said driver unit; detecting a force exerted as areaction force on said driver unit by said melted metal terminalsbetween said semiconductor device and said wiring board during therelative movement of said semiconductor device with respect to saidwiring board; and controlling said driver unit so that said force isobtained as a predetermined constant force, resulting in determinationof an invariable gap between said semiconductor device and said wiringboard.
 20. A soldering apparatus for mounting a semiconductor device ona wiring board, which soldering apparatus comprises: a stage on whichsaid wiring board is placed; a driver unit that holds said semiconductordevice having a plurality of external metal terminals, so that saidsemiconductor device is moved with respect to said wiring board; aheater unit that thermally melts said external metal terminals; and aload sensor that detects a force exerted as a reaction force on saidsemiconductor device by said melted external metal terminals betweensaid semiconductor device and said wiring board during the relativemovement of said semiconductor device with respect to said wiring board;and a control unit that controls said driver unit so that said force isobtained as a predetermined constant force, resulting in determinationof an invariable gap between said semiconductor device and said wiringboard.